The present invention relates generally to semiconductor lasers, and more specifically to coupled resonator vertical-cavity lasers having multiple active gain regions.
Conventional vertical-cavity surface emitting lasers (VCSELs) generally comprise a slab of active laser gain medium sandwiched between a pair of dielectric mirrors, these substructures being monolithically integrated on the surface of a semiconductor substrate. Such devices are usually lattice-matched or strained-layer structures, made of single-crystal direct-gap semiconductor materials. The most common materials used for construction are gallium arsenide based semiconductor alloys.
VCSELs are typically constructed as a vertical stack of layers on a substrate, and are configured so that the laser light they generate is emitted substantially normal to the surface of the substrate. As a result, VCSELs can be effectively used to optically transfer information from one subsystem to another, either via free-space coupling or by coupling to a fiber optic.
Another common application for VCSELs is to form a linear or two-dimensional matrix of laser emitters. Such a matrix can have close (xcx9c100 micron) spacing, enabling very dense matrices having a large total number of emitters. VCSELs are also used for many other applications for which edge-emitting semiconductor lasers are not well suited.
There are many applications for which the performance of lasers can be improved from that of the simple VCSEL described above. An approach known to be useful for improving performance of edge-emitting lasers is the cleaved-coupled-cavity (C3) laser.
A C3 laser is formed as a conventional edge-emitting laser, but following fabrication, a portion of the laser structure is separated by cleavage perpendicular to the laser emission axis. That portion is separated from the active laser device by a small distance along the laser emission axis, so that the additional optical boundary conditions associated with the additional optical cavity and with the surfaces between the active laser device and the additional optical cavity result in easier single-mode operation and in a narrower laser linewidth.
In practice, C3 lasers have limited usefulness owing largely to thermal drift, which alters the resonant wavelength of the various portions of the device. In addition, it is difficult to precisely construct C3 lasers to exhibit a preselected combination of properties, owing to the need to cleave or polish the portions to a length accurate to the order of 100 angstroms, and the need to handle and reposition the portions with a separation having similar accuracy. Vertical alignment of the cavity structures can also be difficult, and the resulting devices exhibit the usual limitations shared by all edge-emitting lasers.
A new class of coupled-resonator vertical-cavity lasers has been invented. These lasers comprise vertically stacked multiple regions of electrically-pumped laser gain media, each such region being located in optically coupled resonators. Each region of laser gain media can be independently injected with current, resulting in an operating parameter space with distinct regions of optical behavior.